In U.S. Pat. No. 3,895,054 there is disclosed a process for the manufacture of urethanes (carbamic acid esters) by reacting an organic compound containing at least one hydroxyl group, e.g. ethyl alcohol, with carbon monoxide and a nitrogenous organic compound at elevated temperature and pressure in the presence of a selenium catalyst and a base and/or water.
The present invention is directed to a simple and effective method of recovering the selenium from urethane solutions produced, for example, by the above described process and containing selenium which may be in the form of selenium per se, inorganic selenium compounds and organoselenium compounds. Because of the high reactivity of selenium, when used as a catalyst as in the above reaction its combination with organic compounds very frequently results in the formation of organoselenium compounds, such as benzoselenazol and diethyl diselenide, which remain in solution with the urethane reaction product. In such reaction, a portion of the selenium catalyst such as selenium per se or selenium dioxide, selenium disulfide, selenium oxychloride, etc. is converted to one or more organoselenium compounds which may be classified generally as selenols, selenides, benzoselenazoles, esters of selenocarbonic acid, selenic acid and selenious acid, selenones and the like. The type and number of organoselenium compounds which may be formed is a function of the reaction conditions used to produce the urethanes such as time, temperature, pressure and solvent. In addition, the inorganic selenium compounds used as catalysts or formed in the reaction may also remain in solution with the urethane product.
Because of the cost and toxicity of selenium, it is essential that as much of the selenium be recovered from the urethane reaction product as is possible and from the inorganic or organoselenium compounds in a form suitable for reuse as a catalyst.
Prior art processes have been proposed for the recovery of selenium from certain organic reaction products and aqueous or acidic solutions. However, such prior art processes are generally narrow in scope and application and have proven to be of little or no value to the recovery of selenium from urethane solutions containing same.
In an article by H. Hauptmann and W. F. Walter, Journal of American Chemical Society, Vol. 77, pp. 4929-4930, Sept. 20, 1955, the action of Raney nickel on organoselenium compounds to effect deselenization is described. A large excess of the Raney nickel is necessary accompanied by long reaction times resulting in partial deselenization.
U.S. Pat. No. 2,671,049 discloses a process for the removal of cyclic aliphatic sulfides from petroleum oils by contacting the oil with mercuric salts to precipitate a cyclic sulfides/mercury salt complex.
U.S. Pat. No. 3,577,216 relates to a process for the recovery of selenium IV used as a catalyst in the oxidative production of carboxylic acids by adding to the reaction products magnesium, zinc, aluminum, alkali and alkaline earth metal hydroxides, oxides, carbonates, bicarbonates and salts of the metals to precipitate metal selenite and reacting the metal selenite with a strong inorganic acid solution to obtain one of selenious acid and anhydride.